In a printer that utilizes a ribbon (heat transfer film, ink ribbon, etc.), the ribbon and a paper sheet are fed in pressure contact between a head and a platen. If the tension of the ribbon varies as this is done, the feed of the ribbon is not concurrent with the feed of the paper sheet, so that the ribbon may be caused to rub against the paper sheet, thereby spoiling the print quality, in some cases. Various proposals have been made to solve this problem (see Japanese Patent Application Laid-Open No. 62-113581, Japanese Utility Model Application Laid-Open No. 2-99657, and Japanese Patent Application Laid-Open No. 11-342661).
In the invention described in Japanese Patent Application Laid-Open No. 62-113581, tension arms that are urged in a loosening direction of the ribbon and detectors for detecting the displacements of the tension arms are located between a ribbon supply shaft and a printing section (in a ribbon supply path) and between the printing section and a ribbon take-up shaft (in a ribbon take-up path). If displacements of the tension arms in the ribbon loosening direction are detected, a ribbon supply shaft drive motor is stopped or a ribbon take-up shaft drive motor is driven. If displacements of the tension arms in a ribbon tensioning direction are detected, on the other hand, the ribbon supply shaft drive motor is driven or the ribbon take-up shaft drive motor is stopped.
In the invention described in Jpn. UM Appln. KOKAI Publication No. 2-99657, an acting body that touches a ribbon and moves up and down as the ribbon tension changes and a sensor that detects a displacement of the acting body in a fixed position are provided in place of the tension arms. If the displacement of the acting body is in a loosening direction of the ribbon, a ribbon supply shaft drive motor is stopped or a ribbon take-up shaft drive motor is driven. If the displacement of the acting body is in a tensioning direction of the ribbon, on the other hand, the ribbon supply shaft drive motor is driven or the ribbon take-up shaft drive motor is stopped.
In the invention described in Japanese Patent Application Laid-Open No. 11-342661, a rotary encoder is provided for detecting the rotational angle of a ribbon core. The outside diameter of the ribbon is calculated from the number of steps of a stepping motor for conveyance and the number of output steps of the rotary encoder that are obtained when a label sheet and an ink ribbon are conveyed a predetermined distance by the stepping motor. The speed and torque of a ribbon driving DC motor are controlled in accordance with the calculated outside diameter of the ribbon.
In the inventions described in Japanese Patent Application Laid-Open No. 62-113581 and Japanese Utility Model Application Laid-Open No. 2-99657, a tensioned state of the ribbon is detected by the tension arms, and the ribbon supply shaft drive motor and the ribbon take-up shaft drive motor are driven or stopped based on outputs of the detectors that are turned on or off depending on the detected tensioned state. Although the construction is simple, therefore, the ribbon supply shaft drive motor and the ribbon take-up shaft drive motor cannot sharply start or stop rotation if they are DC motors. Thus, frequent repetition of drive for ribbon supply or drive for ribbon take-up cannot be dealt with, in some cases. Since the respective diameters of a ribbon supply roll and a take-up roll change, moreover, the motor speed for one cycle of take-up operation must actually be adjusted on each occasion, so that effective control is difficult. For this reason, furthermore, the ribbon supply shaft drive motor and the ribbon take-up shaft drive motor require a performance such that they can produce high starting torque despite the availability of low speed when the roll diameter is large and a performance such that they can start at high speed despite the availability of low starting torque when the roll diameter is small. In case where no mechanism is provided for directly detecting the roll diameter, however, a motor that meets those two requirements, that is, a motor that can start at high speed and also can produce high starting torque, is needed, but such a motor is expensive naturally.
In the invention described in Japanese Patent Application Laid-Open No. 11-342661, on the other hand, the ribbon diameter cannot be calculated unless the ribbon is conveyed a predetermined distance immediately after connection to the power supply or paper or ribbon replacement, although precision control can be achieved in principle. Therefore, the detection of the ribbon diameter takes time, so that the ribbon is used wastefully. If the feed rate is low or if forward feed and backward feed are repeated, moreover, the roll diameter may possibly fail to be detected or the detection may be inaccurate.
Accordingly, the object of the present invention is to provide a printer using a ribbon supply shaft drive motor and a ribbon take-up shaft drive motor, which are relatively low-cost and are expected only to be able to produce a torque that can be activated when a ribbon roll has its maximum diameter, and besides, being capable of practically precisely controlling the degree of tension of a ribbon.